Decarboxylation of Multi-fluorobenzoic acids in NH3-Enriched High Temperature Liquid Water

Multi-fluorobenzenes and multi-fluorobenzoic acids are both important fluorinated fine chemicals which are widely used in the synthesis of pharmaceutical, pesticide and liquid crystal material intermediates.  The decarboxylation of tetrafluorophthalic acid, 2,3,4,5-tetrafluorobenzoic acid and 2,4,5-trifluorobenzoic acid in NH₃-enriched high temperature liquid water (HTLW) was carried out.  Their corresponding products were 2,3,4,5-tetrafluorobenzoic acid, 1,2,3,4-tetraflaorobenzene and 1,2,4-trifluorobenzene, respectively.  The effects of ammonia concentration, initial concentration of reactant, reaction temperature and different metal catalysts were then investigated.  For the decarboxylation of tetrafluorophthalic acid, the conversion and selectivity can be achieved more than 90% without additional catalyst.  The presence of ammonia could remarkably accelerate the decarboxylation rate, but had no significant effect on the selectivity of decarboxylation. The kinetics data at different temperatures were fitted by the first-order equation, and the apparent activation energy of the reaction obtained by Arrhenius plot was 190±11 kJ/mol.  For the decarboxylation of 2,3,4,5-tetrafluorobenzoic acid and 2,4,5-trifluorobenzoic acid, the addition of ammonia and copper based catalysts could improve the reaction rates and selectivities, and the selectivities to pentafluorobenzenes were around 90%.  Nano copper powder and cuprous oxide were proved to be the most effective for the decarboxylation of these fluorobenzoic acids.  Summing up the experiment results on the decarboxylation of multi-fluorobenzoic acids, the catalysis mechanism was well demonstrated.